Anadarko and its partners, ENI, Maersk and Sonatrach, have been active for over 15 years in the Ghadames Basin. This evaluation, based on the predictive methodology developed by J.J. Arps and T.G. Roberts and applied to public domain and company data, predicts that the potential of that Paleozoic-source, Triassicreservoir petroleum system in the Ghadames Basin is less than one billion barrels of oil reserves yet to be discovered. It also predicts that all those reserves will be distributed in fields with reserves of less than 500 million barrels each.

The Paleozoic-source, Triassic-reservoir petroleum system of the Ghadames Basin of North Africa extends from eastern Algeria, across southern Tunisia and into western Libya. Oil and gas in Triassic reservoirs have been correlated through the analyses of geochemical markers to have migrated primarily from Silurian and Devonian mudstones. Migration from the Paleozoic subcrop that was uplifted by the Hercynian Orogeny into overlying Triassic reservoirs is easy to envisage and probably very efficient, but there is also evidence for migration vertically up faults and for long distances along Devonian and Silurian sandstone carrier beds. Uneven subsidence in the Triassic provided the accommodation space for the preservation of a semi-arid fluvio-deltaic clastic system that thickened from the western and northern edge of the basin toward Tunisia. The

Triassic-reservoir petroleum system.

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traps are structural and subtle but there is also evidence that the column heights are enhanced by stratigraphic components. The subtlety of the traps belies the fact that they have undergone a polyphase deformation history that has influenced sedimentation patterns in the Mesozoic. Despite the complicated Tertiary tectonic history, the seals remained intact. The Triassic reservoirs are capped by a mega-regional salt seal deposited in the Jurassic and Cretaceous.

In its world assessment of hydrocarbons in 2000, the U.S. Geological Survey estimated 4.4 billion barrels of liquid petroleum (crude oil plus natural gas liquids) and 12 tcf of gas remains in the Ghadames Basin. Our estimate would place the liquid potential at no more than 1 billion barrels. The Paleozoic-source, Triassic-reservoir petroleum system in the Ghadames Basin is now adequately explored to be assessed on the basis of its past performance, particularly in view of the high levels of seismic and drilling activity over the last decade. The Arps and Roberts method draws on past drilling results to predict the ultimate potential and the field size distribution. Our understanding of the geology and unique data-set of the basin also helps to validate input data and to calibrate the results. The methodology predicts limited additional conventional oil potential in the Ghadames Basin. The liquid potential expected to be found in the next 30 years is about 1 bbo to be exploited in fields with a range of about 50–200 mmbo.

The liquid potential of the deeper Paleozoic-source and Paleozoic-reservoir petroleum system is limited. The basin has significant gas potential for which we make no estimates. From the number of exploratory wells and discovered fields, the deeper Paleozoic petroleum systems could be considered underexplored. However, over most of the basin the older reservoirs are most likely too tight due to their depth and also highly fractured. Much of the producible resource is most likely gas and distributed in numerous relatively small accumulations.

Incremental Reserves Drilling 150 Wildcats.

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